Nitric acid oxidizes iodine to iodic acid, $\ce{HIO3}$ and not periodic acid, $\ce{HIO4}$, which is of a higher oxidation state. This happens although other nonmetals are oxidized to their highest oxidation state. How can this be explained?

  • $\begingroup$ Can u pls give a balanced chemical equation for the reaction of I2 with conc HNO3. $\endgroup$
    – Yashwini
    Commented Aug 21, 2020 at 18:34
  • $\begingroup$ @Yashwini Sorry. This was a long time back. I am majoring in a completely different field for more than three years $\endgroup$ Commented Aug 21, 2020 at 19:23

1 Answer 1


Nitric acid is not strongly oxidizing enough to get iodine to +7. Relativistic effects tend to make the $s$ subshell valence electrons more stable than the others in heavy atoms1. So to some extent, iodine acts as if it has only five valence electrons. To pull out those last two electrons and get periodic acid, electrochemical oxidation is commonly used with a very high oxidation potential.


  1. Relativistic Effects in Chemistry: More Common Than You Thought, Pekka Pyykkö, Annual Review of Physical Chemistry 2012 63:1, 45-64 (link)
  2. https://en.wikipedia.org/wiki/Periodic_acid
  • $\begingroup$ The equation is : $\ce{3 I2 + 10 HNO3 -> 6 HIO3 + 10 NO + 2 H2O}$ $\endgroup$
    – Maurice
    Commented May 15, 2021 at 9:32
  • $\begingroup$ Nitric acid isn't strong enough, but ozone is, isn't it? Still, it doesn't oxidises it to periodic acid... can you explain why? $\endgroup$
    – V.G
    Commented Jun 4, 2021 at 14:40
  • $\begingroup$ It may be easier to get iodine(VII) by using an alkaline medium. $\endgroup$ Commented Jun 6, 2021 at 0:11

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